From the very outset Darwin’s extensive use of metaphor in the Origin has proved controversial, with some people thinking Darwin was thereby committed to ascribing intentions or even consciousness to nature, and others fearing that readers would be misled into thinking that he was. Also, some have argued (e.g. Gillian Beer) that Darwin should be regarded as much as a poet as a scientist. We argue that, on the contrary, his metaphors have a substantively scientific role, and do real work in the development of his argument. Firstly, as Darwin himself stresses, ‘such metaphorical expressions… are almost necessary for brevity’. Secondly, they provide a method for forming new concepts (as in the case of ‘struggle’). Thirdly, and, most significantly, the use of metaphor enables Darwin to explore further the analogy between NS and AS and directly compare the achievements of human breeding and those of the struggle for existence.

Our task here is to address four authors who have given different accounts of Darwin’s argument from ours: Richard A. Richards; Peter Gildenhuys; James Lennox; and D. Graham Burnett. Viewing analogical argumentation as hopelessly unclassy, each has sought to save Darwin’s reputation by denying that he founded his theory of natural selection on an analogical argument, and by offering alternative, non-analogical readings of Darwin’s argumentation. For Lennox, Darwin met the adequacy requirement of the vera causa tradition not through analogy but through speculative conjectures: “Darwinian thought experiments,” Lennox calls them. For Richards, the Origin should be read as an experimental report, in which artificial selection is the cause of new domesticated varieties that periodically go feral, allowing us, as the varieties return to the wild state, to observe the effects of natural selection in action. Explaining why these revisionist accounts cannot be accepted will confirm our explicit views about analogical argumentation, and some implicit ones about relating texts and contexts.

The concept of analogy was first analysed in classical Greek thought. By 'analogy' was meant a four-term relation: A is to B as C is to D. Initially, within Greek mathematics, analogy expressed the equality of the relative magnitudes of two line pairs, when the ratio of line A to line B is identical with the ratio of line C to line D. An analogy asserted a proportionality. And the theory of similar triangles exhibits the basic form of argument by analogy, with a set of valid proofs showing which additional properties, equiangularity say, the two triangles must share. In Euclid are all the features of the analogical relationship relevant to our enquiry. For analogy was soon taken beyond its mathematical confines, especially by Aristotle, in exploring how these geometrical concepts can be applied in empirical contexts. These explorations kept the commitment to proportionality, which persists in every modern analyst of analogy knowingly upholding the Aristotelian tradition.

Central to this book is a trio of chapters (4, 5 and 6) on Darwin’s Origin of Species in its first edition of 1859. Darwin called his book 'one long argument'. These three chapters clarify how this long argument is conducted; how Darwin’s analogical reasonings about natural and artificial selection support the argument; and how his various metaphors are grounded in those reasonings. The conclusions from these chapters support the claims in our chapters 1 to 3 about the decisive antecedents, from ancient times on, for Darwin’s conception of analogical reasoning and what it can do for his one long argument. Equally these conclusions support the claims made in our chapters 7and 8 as to how that reasoning should be analysed and evaluated by philosophers, biologists and historians today. Our writing combines throughout narratives that are often not overtly normative with judgements that often are so.

In the decades before the Origin, a split arises between two very different concepts of analogy, and so two views of argument by analogy. Some people, taking 'analogy' as a synonym for 'similarity',came to a new understanding of 'argument by analogy': suppose A and B are known to share a number of properties, then the probability is increased that B also possesses some other property which A is known to possess. This account remains widely assumed even today. Other people, largely within Anglican theology and concerned with the analogy between God and the world, insisted that the only correct use of the word 'analogy' was in its original Greek sense, including the Aristotelian commitment to proportionality, and so to relational comparisons, as when God is related to his creatures as a human father is to his children. This commitment grounded a view radically different from the new similarity view. In analysing what an argument by analogy is, Richard Whately, and following him J. S. Mill, specified explicitly the conditions for such an argument to be valid. It is this account that is relevant to an understanding of Darwin’s use of analogy.

This chapter engages two clusters of long-run, big-picture issues. One concerns relations between art and nature. Aristotle’s views on this were challenged in the late seventeenth century by Robert Boyle in defending the new mechanical philosophy. Darwin is aligned with neither Aristotle nor Boyle; nor with German Romantic philosophers, such as Schelling. The agrarian contexts of Darwin’s science, and its alignments with agrarian rather than industrial forms of capitalism, illuminate Darwin’s views, including his natural theological views, of art-nature relations. A second cluster of issues concerns the role of the selection analogy in later controversies about natural selection, notably involving Alfred Russel Wallace and Francis Galton in the nineteenth century, and Ronald Fisher and Sewall Wright in the twentieth century. We stress that Darwin’s theorising is sometimes ancient in its resources and sometimes modern, which is not surprising given the intellectual life he was leading. His analogical argument belongs in the science classroom not because it is up-to-date but precisely because, like all science, it is of its time.

Against this background, we turn to Darwin himself. We first look at the selection analogy in his theorising before writing the Origin. Darwin arrived at his theory of natural selection before contemplating such an analogy. We cannot, then, understand the analogy as what led him to the theory. Its role was to support a theory already arrived at. The evolutionary process takes place over millions of years at an imperceptibly slow pace, and so is inaccessible to direct observation. However, here and now we can observe the selective human breeding of domestic animals and cultivated plants. Darwin can then use an argument by analogy to give his theory indirect empirical support. The struggle for existence in the wild and the human breeders are not intrinsically similar agencies, but are relationally comparable in having the same kind of causal relation to the animals and plants that they are acting on with effects similar in kind but not in degree.

There are conditions satisfied by successful analogical arguments which Darwin’s argument satisfies. Darwin first establishes that breeding practices are an analogical model of the struggle for existence in the wild: just as humans discriminate in favour of animals and plants with desirable traits, so the struggle for existence discriminates in favour of creatures with traits best enabling them to cope with that struggle. Domestic breeding creates new varieties because it is systematic – there will be a tendency always to discriminate in favour of the same set of traits. The struggle for existence will have the same systematic tendency to favour certain traits at the expense of others. Therefore it is possible for it also to create new varieties. Darwin now alternates the analogy and its proportionality: if natural selection (NS) is to new wild varieties as artificial selection (AS) is to new domesticated varieties, then NS is to AS as new wild varieties are to new domesticated varieties. Thus if NS is a massively more efficient selector than AS, and the greater the cause, the greater the effect, then, a fortiori, NS should produce not only new varieties but new species.

Here we examine how the Origin deploys the selection analogy. Darwin works in a vera causa tradition. To discover the true cause of some phenomenon, one first establishes that the cause exists (an existence case); then shows that it has the power to produce the effect (an adequacy case); and finally that it has in fact done so (a responsibility case). After examining how breeding practices produce new varieties, Darwin establishes the existence of natural selection. In the wild, resources are limited, and so there is a struggle for existence, with the individuals with the most favourable traits most likely to survive. Next comes the argument showing that natural selection should produce new species, that it is causally competent and adequate. This argument includes reasoning a fortiori: natural selection, being far more comprehensive, precise and prolonged than artificial selection, is a much greater power and so able to produce the adaptive diversification of distinct species over eons of time. Darwin, in later chapters, offers evidence that natural selection has in fact been responsible for producing new species, new genera and even new classes of animals and plants.